SAN FRANCISCO - Scientists have genetically engineered a dozen
cows to be free from the proteins that cause mad cow disease, a
breakthrough that may make the animals immune to the brain-wasting
disease.

An international team of researchers from the U.S. and Japan
reported Sunday that they had "knocked out" the gene responsible
for making the proteins, called prions. The disease didn't take
hold when brain tissue from two of the genetically engineered cows
was exposed to bad prions in the laboratory, they said.

Experts said the work may offer another layer of security to
people concerned about eating infected beef, although though any
food derived from genetically engineered animals must first be
approved by the Food and Drug Administration.

"This research is a huge step forward for the use of animal
biotechnology that benefits consumers," said Barbara Glenn of the
Biotechnology Industry Organization, a Washington industry group
that includes the company that sponsored the research as a member.
"This a plus for consumers worldwide."

The surviving cows are now being injected directly with mad cow
disease, known as bovine spongiform encephalopathy, or BSE, to make
certain the cattle are immune to it.

Those key results won't be known until later this year, at the
earliest, according to the Sioux Falls, S.D. based biotechnology
company Hematech Inc. that sponsored the research. It can take as
long as two years for mad cow disease to be detected in infected
animals.

The research published in the online journal Nature
Biotechnology could be used as a tool that would help researchers
better understand similar brain-wasting diseases in humans, Glenn
and others said.

Scientists are still mystified by the biological purposes of
normal prions, which humans also produce. But they believe that
even one prion going bad can set off the always fatal and painful
brain disease - known as variant Creutzfeldt-Jakob disease in
humans.

Similar prion-based diseases also are found in sheep, deer and
elk.

Glenn and others stressed that the mad cow threat to the United
States is extremely low due in large part to government regulations
enacted after outbreaks in Europe.

"At the moment we don't have a high threat of BSE," said Val
Giddings, a scientist who consults with biotechnology companies.
"But if BSE were ever to become a problem, this could turn out to
be a good technological fix to it."

Also, Hematech's chief scientist, James Robl, said companies
still are spending millions of dollars annually to protect their
cows from the disease.

In the lab, Robl and his colleagues, who included a scientistfrom the U.S. Department of Agriculture, scraped skin cells fromcows and "turned off" the gene that makes prions.

Then, using those cells as a "starter kit," they produced 12
calves through cloning processes - the fusing of the cells into the
eggs of cows. Three were slaughtered so their brains could be
studied and nine are still living.

"The cloning process itself is very large scale," said Robl, who
estimated that Hematech implants about 15,000 cloned embryos into
4,000 cows annually. Most of the pregnancies are terminated before
birth to collect cells for the company's research in developing
human medicines, he said.

Robl said a more immediate use of the technology could be to
produce prion-free cows to produce cow serum, a popular laboratory
tool used for myriad biological experiments.

Since three cows in the United States were diagnosed with BSE
beginning in December 2003, most labs order their cow serum from
New Zealand.

But Hematech isn't much interested in producing serum for
scientists and has no plans to become a beef producer.

Instead, the company is genetically engineering cows to produce
antibiotics and other medicines for people.

The company embarked on the mad cow disease project five years
ago to ensure it could produce medicines that were free from the
brain-wasting disease. BSE is caused when one misshapen prion
prompts normal prions to turn bad, slowly boring lesions in the
brain and making infected animals go mad.

It's thought that people eating infected beef can contract the
human variant of the disease, which also occurs spontaneously.

At least 180 people worldwide have died after eating meat
infected with mad cow disease in the last two decades. Symptoms can
take years to develop.

But scientists are certain the brain-wasting diseases are caused
by the misshapen prions, one of the most mystifying particles in
biology. No one knows the function of normal prions and the
research published Sunday suggests the proteins have little
value.

All the prion-free cows the research team created were born
healthy, although Robl noted that since they are only two years old
they will have to be watched to see if the lack of prions has any
future health effects.